.v-tree-checkboxed .v-tree-node-caption-unchecked div span {
    background: url("img/checkbox-unchecked.png") no-repeat;
    padding-left: 24px;
}

.v-tree-checkboxed .v-tree-node-caption-checked div span {
    background: url("img/checkbox-checked.png") no-repeat;
    padding-left: 24px;
}